As portable electronic equipment has become smaller and thinner, the mounting density of electronic components, such as semiconductor devices, resistors, capacitors, on PWBs has increased remarkably. Multilayer PWBs enable high-density mounting of such electronic components. A multilayer PWB, for example, is formed by laminating plural PWBs on which conductive circuit patterns are previously formed, and connecting the circuit patterns of the PWBs with each other by a via hole.
Published Unexamined Japanese Patent Application (Tokkai-Sho) No. 60-57999 discloses a multilayer wiring board, where the inner layer patterns of the laminated PWBs are partially exposed in step-wise, and a conductive pad is connected to the exposed conductive patterns. In a general multilayer wiring board, a conductive pad is provided only to the outer layer, therefore, many via holes are necessary to connect the exterior patterns and the inner layer patterns. On the other hand, the multilayer wiring board disclosed in this reference can reduce the number of the via holes by providing another conductive pad to the exposed portion of the inner layer. This reference, however, does not disclose how to connect the conductive pad provided to the exposed portion of the inner layer to the electronic component, or how to produce such a multilayer wiring board.
Published Examined Japanese Patent Application (Tokkoh-Hei) No. 5-41039 discloses a PWB, where a recess is provided by counterbore processing, and a semiconductor device is placed in the recess. The semiconductor device and the conductive pad of every layer are connected by a bonding wire, and sealed with resin. In this way, the semiconductor device is contained without substantially exceeding the thickness of the PWB. This method depends on connection by the bonding wires, and thus, this reference does not disclose application of the method to a surface mounting technology (SMT).
FIG. 16 is a cross-sectional view to show the PWB 31 to which a component 32 such as an integrated circuit is mounted by conventional surface mounting technology. Since a lead pin 32a of the component 32 is soldered to the conductive pad 33 on the PWB 31, the component 32 is electrically connected and fixed to the PWB 31.
FIG. 17 shows an example where a component 34, such as a speaker, is mounted on the surface of the PWB 31. The component 34 is fixed on the PWB 31 by an adhesive tape 35, while the lead wire 34a from the component 34 is soldered to the conductive pad 33 on the PWB 31.
In a PWB assembly using SMT, the mounting area is reduced due to the high density trend, however, the assembly has not decreased significantly in thickness. Especially, when a comparatively large component such as a package semiconductor (e.g. IC, LSI), a speaker, or a microphone is mounted, the component becomes much higher than the surface of the wiring board.
Therefore, an assembly using a conventional multilayer PWB does not meet the requirements for thin portable electronic equipment. If the assembly is thick, the portable electronic equipment cannot be thin, and the design of such electronic equipment will be limited. Reduction of the assembly's weight is another important aim.